The present invention relates generally to proximity sensors and particularly to proximity sensors based on sensing changes in a radial magnetic field in the plane of the sensing element.
There has been, and continues to be, a need to do better magnetic detection of the location or proximity of a ferromagnetic object relative to a magnetic sensor. The need exists in a variety of commercial, industrial and automotive applications.
The need for better position sensing is especially great in applications requiring the sensing of the motion of the teeth on a gear which are moving near the end of a probe type sensor. These needs include, for example automotive sensing applications such as engine cams and anti lock braking systems.
A number of proximity sensing devices based on magnetic sensing have been utilized in the past. Variable reluctance sensors (VRS) have been used in which the motion of gear teeth causes the net flux from a permanent magnet to create a voltage in a coil wrapped around the magnet. For lower gear teeth speeds the voltage generated is too small to be detected above the ambient noise.
Magnetoresistive sensors have been proposed which have a single magnetic sensitive axis. These are used in conjunction with a permanent magnet to provide a magnetic field which is then modulated by the position of the ferromagnetic object, e.g., the gear teeth moving nearby.
Hall type magnetic sensors which include a permanent magnet and a Hall effect transducer have been used in gear tooth sensing applications. When a gear tooth is not near the transducer face a fixed level of magnetic field is detected by the transducer. When any tooth is moved into proximity to the sensor, the tooth acts as a flux concentrator to increase the field detected. This increase in the detected field relative to the fixed level is small and this results in a very low level output signal.
Thus a need exists for a proximity sensor which has a larger output signal level than past proximity sensors and that has an output signal level that is independent of the frequency at which gear teeth pass the sensor